A continuing program in laboratory animal nutrition involves various stocks and strains of rats, mice and guinea pigs as well as various species of nonhuman primates. A series of factorial designed feeding trials are used to ascertain the nutrient requirements of the animals involved and to develop diets with more nearly optimal nutrient concentrations for their growth, reproduction, maintenance or general health status. This program has resulted in the development of open formula natural ingredient diets for various species of small laboratory animals which are currently being used throughout NIH and the entire biomedical research community. Diets most recently developed are for nonhuman primates and toxicology studies with mice and rats. Currently work is continuing to develop diets for marmosets and for maximum reproduction in the inbred guinea pig.